1. Field of the Invention
The present invention relates to an improvement of an internal combustion engine variable compression ratio system, in which a compression ratio of an internal combustion engine is changed over between a high compression ratio and a low compression ratio with a changeover threshold value corresponding to a predetermined operational condition of the internal combustion engine being a boundary.
2. Description of the Related Art
Japanese Patent Application Laid-open Nos. 2005-54619, 64-15438, 9-228858, and 60-142020 disclose various types of an internal combustion engine variable compression ratio system which enhances combustion efficiency to increase output and reduce fuel consumption by changing over the compression ratio of the internal combustion engine corresponding to the operational condition, while avoiding abnormal combustion such as knocking.
Also, in the variable compression ratio system disclosed in Japanese Patent Application Laid-open No. 64-15438, in the case where changeover hunting is predicted in the system when the system enters a state in which the operational condition of the internal combustion engine frequently crosses the changeover threshold value (mainly caused by load fluctuations), such as in the case of slow acceleration or deceleration, changeover is prohibited for a certain period of time so as to hold the compression ratio before the occurrence of changeover hunting, whereby the durability of the system is secured.
However, the variable compression ratio system which avoids the changeover hunting by prohibiting the changeover for a certain period of time to maintain the compression ratio before the occurrence of changeover hunting when the changeover hunting is predicted as described above, cannot maintain a compression ratio appropriately corresponding to traveling characteristics demanded by the driver. Therefore, improvement in drivability, output, and mileage is not satisfactory.